feat(supervisor): RealChild + spawn_with_logs

Append RealChild (real tokio::process::Child wrapper) and spawn_with_logs
to child.rs. Uses nix::unistd::setpgid via tokio's re-exported pre_exec
to create an own process group, and fires per-stream log pump tasks that
drain stdout/stderr into the provided LogSink. terminate/kill signal the
whole process group via kill(-pgid, SIG*).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/xy-supervisor/src/lib.rs

@@ -7,7 +7,7 @@ pub mod policy;
 pub mod retry_window;
 
 pub use backoff::Backoff;
-pub use child::{ChildHandle, MockChild, MockChildController};
+pub use child::{ChildHandle, MockChild, MockChildController, RealChild, spawn_with_logs};
 pub use logs::{LogSink, RecordedLine, RingBuffer, RotatingLogWriter};
 pub use policy::{RestartDecision, decide};
 pub use retry_window::RetryWindow;